There is a need for improved overvoltage protection for electric systems, for example electric systems including VSC-converters (VSC=Voltage Source is Converter), electric systems including line-commutated converters, and/or electric systems including power transmission lines, for example, High Voltage Direct Current (HVDC) transmission systems.
An electric system including a wind power park or wind farm is an electric system where overvoltages can be especially adverse. Conventionally, the wind turbines of an offshore wind farm are connected to a local AC bus which is connected to a first VSC-converter, a second VSC-converter is connected to an on-shore AC grid, and a DC link for transmitting the power generated by the offshore wind farm to the onshore AC grid is connected between the first and the second VSC-converter. A 3-phase fault in the AC-system on the inverter side, which includes said second VSC-converter and the AC grid, will cause high overvoltages on the rectifier side, which includes said first VSC-converter and the DC link, i.e. said first VSC-converter and the DC link will be subjected to adverse overvoltages. As a consequence, the VSC-converter on the rectifier side may not be able to receive the power produced by the wind turbines, and the wind turbines have to be brought to a standstill, and when the fault is rectified, or repaired, the wind turbines have to be started up again, and this process results in substantial fall in power production.
WO 2008/131799 A1 discloses a number of solutions to protect an electric system against overvoltages, the electric system including a wind farm and a DC link for the transmission of power from the wind farm to an AC grid, where the DC link at each end is connected to a VSC-converter. In one solution, a DC chopper, which is composed of a switchable resistor, is connected to the DC link. In another solution, the AC voltage in the local AC bus, to which the wind farm is connected, is modified by controlling the VSC-converter on the rectifier side.
It is known in prior art to use surge arresters to reduce overvoltages in an electric system in order to protect the electric system against overvoltage occurrences. For example, WO 02/50972 A1 discloses a VSC-converter for converting direct voltage into alternating voltage and vice versa, in which at least one of the current valves of the VSC-converter is provided with a circuit for overvoltage protection connected in parallel with the current valve, which circuit comprises a series connection of a surge arrester.
However, when a surge arrester is used to protect an electric system according to prior art, for example the above-mentioned electric system including the wind farm, the protective level, or the knee-point voltage, of the connected surge arrester is adapted to the electric system and set to a fixed maximum operation voltage of the electric system, and this results in an inflexible overvoltage protection.